Recently, telephone sets having a volatile memory which stores information such as telephone numbers or messages have been widely used. Such telephone sets include, for example, telephone sets having an abbreviated dialing function, automatic answering telephone sets, etc.
In a telephone set having an abbreviated dialing function, telephone numbers are registered in a volatile memory in terms of abbreviated telephone numbers of one or two digits. When an abbreviated number is dialed, the telephone number corresponding to the abbreviated number is read from the volatile memory and is sent to the telephone office.
However, the volatile memory is likely to lose its stored data inadvertently. Unless a voltage is continuously applied to the volatile memory, the memory cannot maintain its stored data. If the application of the voltage discontinued, the stored data is lost immediately. In place of the volatile memory, it may use a rewritable non-volatile memory. However, a rewritable non-volatile memory is more expensive than a volatile memory so that a telephone set becomes costly.
Therefore, a telephone set having an abbreviated dialing function generally uses a relatively inexpensive volatile memory (RAM) and a battery as a power source for maintaining the storage in the volatile memory. Such telephone set, however, has a problem that if the voltage of the battery is greatly reduced due to its consumption or the voltage applied to the volatile memory is interrupted temporarily at an occasion of the replacement with a new battery, the data stored in the volatile memory is lost undesirably.
An automatic answering telephone set is also generally uses a volatile memory to store messages to be sent to a caller. When there is an incoming call while the automatic answering function is set in operation, the message stored in the volatile memory is sent to the caller. When a message is received from the caller in response to the sent message the received message is stored in the storage unit provided in the telephone set, such as a tape recorder which uses a magnetic tape as a recording medium so as to be able to reproduce the received message
Since an automatic answering telephone set includes such a storage unit which consumes large power, it uses a commercially available AC power supply service. Therefore, the data stored in the volatile memory is maintained by a voltage from the AC power supply service. In addition, a battery as a backup power source is provided in case that power supply from the AC power supply service is interrupted.
However, such automatic answering telephone set also has a Problem that if an AC power supply service is interrupted for a long time, the battery is consumed and its voltage becomes lowered enough to lose the data stored in the volatile memory To cope with this, if the capacity of the battery as the backup power source must be large. As a result, however, cost is increased and the space factor is degraded.